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Technical Bulletin
Particle Size Analysis in Dry Powder
Cell Culture Media Production
Kurt Sundgren
Process Chemist II DPM...
Particle Size Statistical Analysis                                Responses to Particle Size Variations
Since particle siz...
density decreases as particle size decreases as a
result of entrained air between the fine particles. As
such it is benefi...
                              Particle Siz e vs. S olubility T ime
                        Sod iu m Pho sph a t...
About the Author                                   About SAFC Biosciences
Kurt Sundgren is a Process Chemist II at SAFC   ...
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Introduction to Powder Size Analysis for Cell Culture Media


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A brief introduction into the applications of particle size analysis of dry powder media.

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Introduction to Powder Size Analysis for Cell Culture Media

  1. 1. Technical Bulletin Particle Size Analysis in Dry Powder Cell Culture Media Production Kurt Sundgren Process Chemist II DPM Introduction Product testing is constantly evolving as underlying unreliable below 75 microns.[1] As impact milling can technologies improve. Particle size determination of produce fines in the sub-micron range another comminuted powders – a vital element in dry powder method must be used to characterize the particle cell culture media production – has existed in various size distribution. forms for centuries, but modern improvements in the SAFC Biosciences also uses a Horiba LA-950 accuracy, range and reporting abilities of particle laser diffraction analyzer. This instrument uses Mie size analyzers (PSA) have extended our capabilities scattering theory of edge and particle diffraction significantly. With the added accuracy and ease to calculate particle size from 10 nm to 3000 µm.[2] of use that comes with PSA equipment, the trend A computer algorithm provides a curve of particle has been to incorporate these results into product diameters. Results can be communicated through specifications. Do these specifications really add statistical or graphical means. The Horiba LA-950 does value if there is no scientific backing of the results? not determine particle shape and assumes all particles PSA alone is not sufficient to fully represent the to be spherical. This assumption is critical as particle physical characteristics of a complex media. It shape affects the results, as well as powder behavior. is the empirical evidence of particle size and the The Horiba particle size analyzer requires approxi- effects of changes in particle size distribution mately 1.5 grams of material. Of those 1.5 grams, which are important tools in assessing the handling only a portion is measured. This small test size properties of milled powder. This paper offers leads to variation among samples. To minimize an overview of the particle size measurement sampling and testing errors, proper represen- capabilities of SAFC Biosciences ®; the effects of tative sampling techniques are used and tests are particle size variability; ways to communicate the performed in replicate. Variability between samples distribution in a beneficial manner; and how best is compared to assess variation across an entire lot. to use this powder characterizing tool. The goal is to demonstrate how SAFC Biosciences uses Particle Size Across Lot current technologies in particle size analysis to Control Limits provide a consistent, high-performing media. 1.20 1.10 1.00 Particle Size Analysis Capabilities 0.90 0.80 Currently, SAFC Biosciences uses a Ro-tap ® sieve 0.70 sifter for creating physically separated streams based Ratio d90 to UCL 0.60 0.50 on product classification through mesh screens. 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 The Ro-tap sifter results have value, but the physical Sample Number screening action itself can cause particle size Average d90 ratio UCL LCL reduction and agglomeration of fine particles make it
  2. 2. Particle Size Statistical Analysis Responses to Particle Size Variations Since particle size for a complex media is a The effects of particle size and distribution distribution of diameters, statistics can be used to changes are multiple and no single parameter convey the results. A common method is to use d10, can be used to fully define the powder. The d50 and d90 values based on volume distribution. following aspects are inter-dependent and all That is to say that 10%, 50% and 90%, respectively, have a major impact on physical properties: of the particle size distribution is smaller than the • angle of repose • friability stated diameter. Standard deviations and span calculations are also appropriate when discussing the • flowability • abrasiveness distribution width. The calculated surface area also • material composition • bulk density provides a helpful frame of reference for solubility conditions. Communicating particle size distri- • particle size and • surface characteristics distribution • moisture content bution through statistic expressions alone can fail to report important information such as bimodality, • particle shape • density which can be conveyed through graphical means. • cohesiveness • viscosity • adhesiveness • agglomeration Most of these properties are inherent to the components of the formulation and cannot be changed. For this paper, three significant areas of concern related to particle size will be discussed: material handling properties, Example of Horiba LA-950 Result mixing/segregation and solubilization. Different measurement techniques call for different means of expression, but independent of the method, Material handling properties -- During manufacturing, it is critical to realize one particle size setpoint will be of the material will show differences in handling as the low value. Reporting a d90 will give an idea of the upper particle size changes. These properties are component diameter range, it will not convey the amount of fine dependent. Assuming a recipe is held constant there particles in the sample. Operationally, this might lead to are several variables that can be affected by particle manipulation of the milling system to meet a value that size. Material handling attributes are hard to define, has little meaning. For instance, by increasing the mill but practical experience has provided some tests. speed and obliterating the particles you would meet The terms flowability and floodability are empirical a d90 specification, but the resulting powder would values normally associated with the Carr indices. have been exposed to higher temperatures and may The Carr indices include density measurements, have a dramatic increase in sub-micron fine particles. angles of repose, and compressibility. Through these evaluations, the ease of handling for the powder can be predicted and engineering solutions can be created. SAFC Biosciences performs Hausner ratio tests to predict flowability for the finished powder. The Hausner ratio is the ratio of the tapped density over the free settled bulk density.[3] The general rule is that Hausner ratios greater than 1.25 indicate that the material will flow poorly. For most cell culture media the free bulk
  3. 3. density decreases as particle size decreases as a result of entrained air between the fine particles. As such it is beneficial to keep overall particle size distri- bution as high as possible without putting the material at risk for sifting segregation or poor solubility. These complexities show the problems that may result from establishing an artificially defined maximum particle size. Product milled to ultra-fine particle sizes will be difficult to pour from drums or barrels, create a dusty formulation environment and can require additional Segregation patterns due to different mechanisms from design considerations for proper material handling. Jenike & Johanson [4] Relationship between Hausner ratio and Flowability Another common type of segregation occurs through entrainment of air. Since finer particles tend to have Hausner ratio Flow Character lower permeability than coarse particles they generally 1-1.11 Excellent retain air longer in their void spaces. This results in 1.12-1.18 Good the coarse particles settling beneath the fluidized fine 1.19-1.25 Fair particles. This is called fluidization segregation and 1.26-1.34 Passable is commonly seen with fine particles. Particles less 1.35-1.45 Poor than 50 µm are also susceptible to entrainment in an 1.46-1.59 Very poor air stream, or dusting.[6] The finer particles remain >1.6 Non-flowing suspended in air streams longer and can be scattered away to specific points by secondary air currents. A single particle size setpoint will be ineffective in dealing Mixing/segregation -- Particle size has implications with segregation as both large and small particles have for mixing/demixing properties. Since particle certain tendencies to different types of segregation. size affects flow properties, the ability to achieve homogeneous blends is also affected. Research Solubility has shown that there is a direct correlation between the rate and degree of mixing and flowability.[5] Solubilization of the final product is also dependent on particle size. The smaller the particle diameter in The converse of mixing is segregation and all mixed a given sample, the larger the total available surface powders have a certain amount of likelihood to area for the solvent to act. The more surface area segregate. This can be minimized through manipu- available, the faster the rate of solution. In practical lation of the material, optimizing the process and applications this does have a limit. When the particle improving equipment design. By changing particle size is reduced too far, the media can agglomerate size distribution the tendency of a product to when the solvent is added. Agglomeration increases segregate can be reduced. Sifting segregation the particle size, therefore decreasing the total surface is the movement of smaller particles through a area for the sample and reducing the rate of solution. mixture of larger particles. Experiments have shown that the tendency to segregate through sifting SAFC has performed studies on individual decreases substantially as particle sizes are reduced components propensity to agglomerate at fine below 500 microns. [6] SAFC Biosciences monitors particle sizes. Dissolving 100 grams of Sodium particle size results to ensure the product stays Phosphate Monobasic Monohydrate in 1 Liter of 20°C well below the sifting segregation high risk zone. stirred water at different particle sizes to determine time to solubility (solubility being defined as a lack of visible particles and a turbidity of <0.85
  4. 4. Conclusion Particle Siz e vs. S olubility T ime Sod iu m Pho sph a te Mo no b asi c Mo no h ydra te SAFC Biosciences will continue to use the most 2 up-to-date methods for product characterization. The data that particle size analysis provides is an 1.5 important tool for defining a product. However, the 1 results from particle size testing should be used 0.5 judiciously, and only for adding value to the process. 0 1.1 1 0.9 0.8 0.7 0. 6 0. 5 0.4 0.3 0.2 0. 1 0 Particle size information is a distribution and not Pa rti cl e Si ze(ratio d 10 to m a x. d10) a setpoint. Imposing an artificial high limit specifi- cation would shift the particle size down. This can NTU). Fine particle sizes lead to product agglom- lead to creation of a product that is at high risk for eration which increased the overall solubility time. certain types of segregation, increased temperatures during milling and may have poor solubility due to The critical aspects of media are dependent on agglomeration. Additionally, the material would be particle size but rely on many additional factors difficult to handle and dusty. SAFC Biosciences uses as well, such as particle shape and density. These the particle size information as well as many other attributes are component-driven and are intrinsic powder characteristic tests to identify and optimize to each recipe. As such, the most advantageous the properties of each unique product manufactured. particle size will depend on the recipe, and the unique properties the milled material possesses. References SAFC Biosciences hones the controllable variables 1. US Pharmacopeia Ch. 616. Bulk Density and to deliver a product that preserves flowability, limits Tapped Density the tendency to segregate and maintains solubility. 2. Trivaranus, I. LA-950 Laser Diffraction Technique Ver. 2. Technical Bulletin, May 2009, pp 4. Future Applications for Powder 3. Sato, T. Evaluating Flow Property of Powder by Carr’s Flowability Method Using the Powder Characterization Characteristics Tester. Technical Data Sheet. In the future, expect to see further advances in Hosokawa Europe Ltd. testing technologies that will allow for process 4. Williams, J.C.; Kahn M.I. The mixing and design controls to further improve product segregation of particulate solids of different particle size. Chem. Eng. 1973, pp 19, 269. quality. These developments may include: 5. Jenike & Johanson. Powder Blending, Bulk Powder • In-line, non-destructive particle size analyzers Handling, Blend Uniformity. being used for real time mill controls Solutions/segblend.html, Sep. 2009. • Increased understanding of shear cell testing 6. Carson J.W.; Royal T.A.; Goodwill D.J. Understanding and Eliminating Particle of samples for a more scientific approach to Segregation Problems. Bulk Solids Handling, determining material handling properties Vol. 6, Feb. 1986, pp 2. • Improved modeling for greater understanding of complex comminution behaviors Refining these processes will depend on a fuller understanding of the immensely complex world of defining powder product characteristics. The sheer number of variables and parameters make this a daunting task.
  5. 5. About the Author About SAFC Biosciences Kurt Sundgren is a Process Chemist II at SAFC SAFC Biosciences is a leading provider of cell culture materials and development services for upstream and Biosciences in Lenexa, Kansas. He is responsible downstream processes in the biopharmaceutical industry. for powder property characterization and process Providing an integrated services package in mammalian controls. He has seven years of experience in cell culture media development, along with analytical and regulatory support, SAFC Biosciences employs a wealth milling applications, five of which have been of industry experience and scientific know-how to deliver in cell culture media production. Kurt received reliable, consistent solutions that accelerate customer his B.S. in Milling Science from Kansas State success – from development through to commercial- ization. The unit has a 35-year history as a manufacturing University in 2003 and is currently pursuing partner providing leading biopharmaceutical companies an M.B.A. from the University of Kansas. with the broadest range of highly customized products and services possible. About SAFC: SAFC is the custom manufacturing and services group within Sigma- Aldrich that focuses on high-purity inorganics for high technology applications, cell culture products and services for biopharmaceutical manufacturing, biochemical production and the manufac- turing of complex, multi-step organic synthesis of APIs and key intermediates. SAFC has manufacturing facilities around the world dedicated to providing manufacturing services for companies requiring a reliable partner to produce their custom manufactured materials. SAFC has four focus areas – SAFC Pharma®, SAFC Supply Solutions®, SAFC Biosciences®, and SAFC Hitech® – and had annual sales of $624 million in 2008. SAFC is one of the world’s 10 largest fine chemical businesses. SAFC®, SAFC Supply Solutions®, SAFC Pharma®, SAFC Hitech® and Sigma-Aldrich® are registered trademarks of Sigma-Aldrich Biotechnology L.P and Sigma-Aldrich Co. . 73099-509946 Ro-tap® is a registered trademark of W.S. Tyler Inc. 1010 © 2010 SAFC All rights reserved.